Why do women have stress urinary incontinence?

This article reviews progress made in understanding the causes of stress urinary incontinence. Over the last century, several hypotheses have been proposed to explain stress urinary incontinence. These theories are based on clinical observations and focus primarily on the causative role of urethral support loss and an open vesical neck. Recently these hypotheses have been tested by comparing measurements of urethral support and function in women with primary stress urinary incontinence to asymptomatic volunteers who were recruited to be similar in age, race, and parity. Maximal urethral closure pressure is the parameter that differs the most between groups being 43% lower in women with stress incontinence than similar asymptomatic women having as effect size of 1.6. Measures of urethral support effect sizes range from 0.5 to 0.6. Because any one objective measure of support may not capture the full picture of urethrovesical mobility, review of blinded ultrasounds of movements during cough were reviewed by an expert panel. The panel was able to identify women with stress incontinence correctly 57% of the time; just 7% above the 50% that would be expected by chance alone, confirming that urethrovesical mobility is not strongly associated with stress incontinence. Although operations that provide differential support to the urethra are effective, urethral support is not the predominant cause of stress incontinence. Improving our understanding of factors affecting urethral closure may lead to novel treatments targeting the urethra and improved understanding of the small but persistent failure rate of current surgery.